This application is based on Japanese Patent Application No. 9-252466 filed on Sep. 17, 1997, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a ridge-type semiconductor laser integrated with a spot size converter.
In optical fiber communications, a single optical fiber can transmit a large amount of information. It is therefore desirable to broaden the application field from current trunk line networks to subscriber networks, to local area networks (LAN) and the like. In order to realize this, a lower initial cost is necessary so that optical coupling between an optical semiconductor element in an optical module and an optical fiber is made easy.
In facilitating optical coupling, a semiconductor laser integrated with a spot size converter has drawn much attention. If the spot size of a laser beam radiated from a semiconductor laser is made large, a high coupling efficiency can be realized without using a lens, and, in addition, a position alignment margin becomes large so that a work of optical coupling is simplified.
2. Description of the Related Art
FIG. 8 is a perspective view of a conventional ridge-type semiconductor laser. On a substrate 100 made of n-type semiconductor material, an active layer 101 having a quantum well structure, a p-type cladding layer 102, and a p-type contact layer 103 are stacked in this order from the bottom. The active layer 101 is gradually thinned toward an output plane (front plane as viewed in FIG. 8) in the nearby area of the output plane.
Two continuous grooves 105 are formed extending from the output plane to the opposite reflection plane. The groove 105 extends from the upper surface of the contact layer 103 to the lower surface of the p-type cladding layer 102. A ridge 104 is defined between the two grooves 105. The ridge 104 is gradually broadened toward the output plane in the nearby area of the output plane.
The spot size of light propagating in the active layer 101 in the ridge 104 becomes larger toward the output plane. The broadened area at the output plane of the ridge 104 corresponds to an expansion of the spot size.
As the quantum well layer of the active layer 101 becomes thin, the band gap of the quantum well structure broadens and the nearby area of the output plane of the active layer 101 becomes transparent relative to an oscillated laser beam. Since current injected into this nearby area does not contribute to laser oscillation, a wasteful power consumption increases. Furthermore, if free carriers are generated in the nearby area of the output plane when current is injected into this nearby area, free carrier absorption occurs to increase a loss of laser beam. It is therefore unnecessary to inject current into this nearby area.
In order to suppress current injection into the nearby area of the output plane of the active layer 101, the contact layer 103 in the upper region of the ridge 104 in the nearby area of the output plane is removed and the current injection electrode is not formed.
In order to facilitate optical coupling to an optical fiber or the like, a laser chip is mounted on a mount substrate through junction-down with its upper surface directed to the mount substrate. In order to facilitate to mounting of laser chip on the mount substrate, a lamination structure having a flat surface and the same height as that of the ridge 104 is left on both sides of the ridge 104.
Since the spot size of a laser beam becomes large in the nearby area of the output plane, the ridge 104 is formed to be thick in correspondence with the large spot size. The spot size of a laser beam in the area on the opposite reflection plane side where the active layer 101 has a uniform thickness, is not so large as that on the output plane side. However, the ridge 104 in this area is also formed thick in correspondence with the thickness of the ridge 104 in the nearby area of the output plane.
In order to obtain a single lateral mode of light, the width of the ridge 104 is generally narrowed to about 2 to 3 xcexcm. In contrast with this, its height is set to about 4 to 5 xcexcm so as to match the large spot size. Such an increase in thickness of the ridge 104 results in an increase in resistance of the p-type cladding layer 102, and when large current is injected, heat is generated and the laser performance may be degraded.
It is an object of the present invention to provide a ridge type semiconductor laser capable of suppressing an increase in resistance of the upper cladding layer and making it easy to mount the laser chip on a mount substrate.
According to one aspect of the present invention, there is provided a semiconductor laser comprising: a semiconductor substrate of a first conductivity type having a principal surface, a laser output plane, and a laser reflection plane opposite to the laser output plane; an active layer formed on the principal surface of the semiconductor substrate continuously at least from the output plane to the reflection plane, the active layer being gradually thinned toward the output plane in a region to a certain distance from the output plane toward the reflection plane; a ridge generally of a ridge shape formed on the active layer and made of semiconductor material, the ridge extending from the output plane to the reflection plane and gradually increasing a width toward the output plane in a region to a first distance from the output plane, an upper surface of the ridge in a first region to a second distance from the output plane being set higher than an upper surface of the ridge in a second region other than the first region, and at least a portion of the second region being of a second conductivity type opposite to the first conductivity type; and a mesa structure formed on the principal surface of the semiconductor substrate in areas on both sides of the ridge, the mesa structure having an upper surface defining a virtual flat plane at a position flush with or higher than a highest upper surface of the ridge.
The spot size of a laser beam becomes large in the active layer in the nearby area of the output plane where the active layer is made thin. In correspondence with an increase in spot size, the ridge is made wide and thick. In the area where the spot size is small, the ridge is made thin. Current is injected into the active layer from this thin area, an increase in resistance of the ridge can be suppressed. Since the upper surface of a mesa structure is made in contact with a mount substrate, the semiconductor laser can be mounted reliably on the mount substrate.